This invention relates generally to a condensing heating module adapted for use with a radiant infrared burner for heating a liquid. The heated liquid may be either a heat transfer fluid for use in a space heating system, or water for general domestic or other hot water service. In particular, the invention relates to a heating module which is adapted for use where the condensed and corrosive combustion products formed by a gas or oil-fired burner are present.
Environmental and economic considerations have led to numerous attempts to produce space and water heating systems of increased thermal efficiency. One method for increasing efficiency in a gas or oil-fired heating system is to capture the heat contained in the combustion gases, heat which is lost "up the flue" in conventional systems. Increases in system thermal efficiency are attained by cooling the flue gases produced by the burner, while the gases are still in the system, to a temperature below the dew point of the gases, thus recovering a portion of the latent heat of vaporization as usable heat energy. This is generally accomplished in a liquid heating system by using the cooler liquid entering the heating system to cool the flue gases while at the same time the gases preheat the liquid. Thermal efficiencies in such condensing systems can be in excess of 90 percent.
A simple and economical method of fabrication for producing a heat exchanger is to stamp and assemble two metal sheets together such that a gas flow path is formed between the connected sheets. Heat exchangers fabricated in this manner are termed clamshell heat exchangers and are widely found in conventional heating systems, especially in furnaces. Heat exchangers in such applications are generally made of carbon steel.
The condensate produced from flue gases in a condensing heating system contains a number of corrosive compounds, including sulfuric, hydrofluoric, and hydrochloric acids. Concentrations of as little as ten parts per million of these corrosive agents may severely corrode carbon steel and pit aluminum and copper. The materials used to fabricate components exposed to the corrosive environment in a condensing system must therefore be different and more corrosion resistant than those used in conventional applications. The ideal material or materials used to fabricate such components should have good heat transfer characteristics, adequate strength with minimum thickness, be resistant to chemical attack and be low in acquisition and manufacturing cost. Condensing heat exchangers are now typically fabricated of corrosion resistant steels such as stainless steels in the 300 series. These steels possess most of the attributes of the ideal material but are relatively costly. There have been efforts made to fabricate components of carbon steel coated with a corrosion resistant material, but coatings applied in a liquid or powder form perform poorly, blister, crack or spall during the fabrication process and subject the carbon steel base metal to localized corrosion.